Streams throughout the Pacific continental United States cooled during the past two decades, scientists report in the peer-reviewed Geophysical Research Letters. The scientists report they expected global warming to cause significant warming of stream water, but they were surprised to find more streams cooled during the 1987-2009 study period than those that warmed.

“Observed trends of increasing air temperature and declining stream flow are widely believed to result in corresponding increases in stream temperature. … Based on hypothesized climate impacts, we predicted that we should find warming trends in the maximum, mean and minimum temperatures, as well as increasing variability over time,” the scientists noted in the study.

“These predictions were not fully realized. Warming trends were most prevalent in a small subset of locations with longer time series beginning in the 1950s. More recent series of observations (1987–2009) exhibited fewer warming trends and more cooling trends in both minimally and highly human-influenced systems. Trends in variability were much less evident, regardless of the length of time series,” the scientists explained.